The invention relates to a method for coupling an inverter to an alternating voltage, the inverter comprising a current controller, which controls phase currents and implements reference current.
When an inverter is being used, it is often necessary to couple it to an alternating voltage source. This occurs for example during a speed start of a permanent-magnet machine, a speed start of an asynchronous machine when a rotor thereof has a residual flux, or during synchronization of a network inverter into an electric network. A speed start refers to a situation where an electric machine rotating in an uncontrolled manner is to be controlled by an inverter. In all of the aforementioned situations, the speed start or synchronization can be based on rotation of voltage or flux, and on observation of this rotation, since for example a rotating magnetized rotor is known to produce alternating voltage in a winding of a stator surrounding the rotor.
In a known method for coupling an inverter to an alternating voltage source, the output voltage is set to zero, or a zero phasor is formed for a predetermined period of time, and behaviour of the current is observed. This type of methods are disclosed in EP 0,653,116 B1 for a short-circuit machine, WO 9,534,125 for a network inverter, and EP 0,994,561 A2 for a synchronous permanent-magnet motor. All the publications disclose a method where zero phasors of the inverter generate two or more current impulses, or short circuits, which are used to determine flux direction, frequency of rotation, and amplitude, if required. These methods provide rapid operation if the voltage is high enough.
A problem with the prior art methods is that synchronization calculations are based on individual and momentary current measurements, which give too much significance to interference. This unavoidably results in inaccuracy that is more emphasized at low current values, i.e. at low voltages of the voltage source. Furthermore, as the frequency decreases, frequency determination becomes more accurate and also requires a longer time interval between the short circuits.
The prior art methods are based on observation of current behaviour. In case of an asynchronous machine, this requires data on short circuit inductance, which is an inaccurate parameter and produces more errors in the operation of the method. In connection with an asynchronous machine, the method does not operate quite reliably, wherefore it can only be used in certain applications. Use of the prior art methods further requires special modulation logic in a modulator of the inverter for implementing required zero vectors that enable synchronization.